1. Field of the Invention
This invention relates to a silicone resin-containing emulsion composition having film formability and capable of forming a cured film having mar resistance, weather resistance, and chemical resistance, a method for preparing the composition, and an article having a cured film of the composition.
2. Prior Art
From the contemporary concern about the environmental pollution and the standpoint of a safe working environment, the switch of dispersing media from organic solvents to aqueous systems is required in the paint and coating field.
In compliance with this requirement, coating emulsions obtained by emulsion polymerizing radical polymerizable vinyl monomers, as typified by acrylic resin systems, have been widely used as the base component of coating compositions because they have good film formability and chemical resistance. Such coating compositions, however, suffer from the essential lack of water resistance and weather resistance.
On the other hand, silicone resins obtained by hydrolysis and condensation of silane compounds attract attention as coating compositions because they are capable of forming films having high hardness, weather resistance, water resistance, heat resistance and water repellency. Nevertheless, the silicone resins are not only inferior in film formability, alkali resistance, and film flexibility, but also lack shelf stability because silanol groups have high condensation activity. The silicone resins are generally used as solutions in organic solvents, which suffer from the hazards of potential fire and explosion, toxicity to the human body, and environmental pollution. It is desired to have a silicone resin composition of the emulsion type.
Various attempts as described below have been made to overcome the above-mentioned drawbacks although they are still unsatisfactory.
(a) An aqueous emulsion is obtained by emulsifying an alkoxysilane compound or a partial hydrolyzate/condensate thereof with the aid of various surfactants (see JP-A 213046/1983 corresponding to U.S. Pat. No. 4,552,910, JP-A 197369/1987 corresponding to U.S. Pat. No. 4,648,904, JP-A 115485/1991, and JP-A 200793/1991 corresponding to U.S. Pat. No. 4,999,249). A system obtained by mixing this emulsion with another emulsion resulting from emulsion polymerization of a polymerizable vinyl monomer is also known from JP-A 344665/1994.
(b) An aqueous emulsion is obtained by forcibly mechanically pulverizing and dispersing a solventless solid silicone resin together with a chain-like silicon compound (see JP-A 247434/1995).
(c) An emulsion is prepared by emulsion polymerizing a radical polymerizable vinyl monomer in the presence of a water-soluble polymer resulting from hydrolysis of an alkoxysilane in water in the absence of a surfactant (see JP-A 60098/1996).
(d) A solid particle emulsion of graft copolymer is obtained by hydrolyzing and condensing an alkoxysilane mixture containing a vinyl polymerizable alkoxysilane into an aqueous emulsion containing a solid silicone resin, and adding a radical polymerizable vinyl monomer thereto, followed by emulsion polymerization (see JP-A 209149/1993 corresponding to U.S. Pat. No. 5,405,691 and JP-A 196750/1995).
(e) An emulsion is prepared by emulsion polymerizing a radical polymerizable vinyl monomer into an emulsion, adding an alkoxysilane to the emulsion, and effecting hydrolysis and condensation, thereby introducing a silicone resin into emulsion particles (see JP-A 45628/1991 and 3409/1996 corresponding to EP 757059A).
(f) An emulsion is prepared by emulsion-polymerizing an alkoxysilane containing a vinyl polymerizable functional group together with a radical polymerizable vinyl monomer (see JP-A 9463/1986 and 27347/1996).
In the case of (a), active alkoxy groups are prone to hydrolysis with the lapse of time, forming in the system an alcohol by-product which is an organic solvent. Another drawback is that a relatively low molecular weight polymeric reactant converts into a high molecular weight compound. As a result, the emulsion becomes unstable due to the alcohol by-product and varies in degree of polymerization. That is, the emulsion shows unstable performance. If a large amount of surfactant is used to compensate for such drawbacks, then the coating experiences a substantial drop of film properties including hardness and water resistance. Since the system consists of a silicone resin alone, the drawbacks inherent to the silicone resin are left unchanged. Therefore, film properties are far from satisfactory.
Method (b) is by forcibly emulsifying particles of a solid silicone resin having a softening point together with a chain-like organopolysiloxane terminated with an OH group at each end with the aid of a surfactant. This method has the advantage that no organic solvent is contained. Nevertheless, since the solid silicone resin due to its high polarity is difficultly soluble in a liquid organopoly-siloxane of low polarity, the solid resin must be dispersed by means of a sand grinder mill. It is then difficult to divide the solid resin into fine particles, resulting in the disadvantage that the emulsion lacks stability. Also, since the solid resin has a relatively low silanol group content of 0.1 to 5% by weight, its curing requires a temperature as high as 200 to 300.degree. C., which means inferior workability to the organic solvent system. Due to the combined use of chain-like organopolysiloxane, the cured coating is good in water repellency, but has low hardness leading to poor mar resistance. Thus the coating is not fully durable.
Method (c) is by first simply hydrolyzing an alkoxysilane in water to form a water-soluble resin which does not contain a surfactant, then adding a surfactant thereto, and emulsion polymerizing a radical polymerizable vinyl monomer therein. This method also has the drawback that hydrolysis in water produces an alcohol by-product. The water-soluble silicone resin is partially taken into emulsion particles although a substantial portion thereof is left in water because of its high water solubility. Active silanol groups contained in molecules which are present in water tend to gradually condense, precipitating as a gel-like matter. As a result, the emulsion becomes unstable. Also the silicone resin component and the vinyl polymerizable component are not uniformly distributed in the cured film since their mutual solubility is essentially poor. Then film properties become poor.
In the case of (d), an alkoxysilane is hydrolyzed in water and an emulsion of a corresponding solid silicone resin is then formed therefrom. Utilizing vinyl polymerizable groups contained in the silicone resin as starting points, acrylic chains are grafted by emulsion polymerization. This method also involves the hydrolysis step of an alkoxysilane, which introduces an alcohol or organic solvent into the system. The emulsion stability is thus insufficient. This method intends to produce solid fine particles capable of imparting lubricity and water repellency, and produces composite particles in which a silicone resin serving for these functions is positioned at the center of a nucleus and acrylic chains contributing to the prevention of agglomeration of particles in water and an improvement in dispersion are grafted thereto. It is therefore preferred that both the components are non-uniformly distributed, the silicone resin undergoes full crosslinking within particles, and the silicone resin is rather free of silanol groups and remains inactive. Consequently, it is not envisaged to form a cured film from the emulsion.
Like the foregoing emulsions, the emulsion (e) has the drawbacks that since an alkoxysilane is post added to an emulsion of vinyl monomer, an alcohol by-product forms and the alkoxysilane is not completely taken into emulsion particles. The stability of the emulsion and the characteristics of a cured film are far from satisfactory. Owing to the post addition of an alkoxysilane, there is a possibility that more silicone resin is present on the outer shell side. Both the components are non-uniform within particles and in cured film. The mutual compensation of both the resins is thus insufficient.
In emulsion (f), an alkoxysilane having a vinyl polymerizable functional group is emulsion polymerized together with a radical polymerizable vinyl monomer. The alkoxy groups are restrained from hydrolysis and left intact. This suppresses the formation of alcohol by-product and changes of characteristics with time as found in the foregoing emulsions. However, it is difficult to introduce a large amount of the silicone resin component into a coating. Thus, the coating is not improved in weather resistance to a satisfactory level for exterior application.
In summary, the prior art well-known methods have not succeeded in providing a silicone resin-containing emulsion having advantages including (i) the absence of highly flammable alcohols having a low boiling point and other organic solvents, (ii) good emulsion stability, (iii) low temperature curing, and (iv) formation of a uniform cured film having improved characteristics including mar resistance, weather resistance, and chemical resistance.